Internet Overview: roadmap

1.1 What is the Internet?

1.2

Network edge  end systems, access networks, links 1.3 Network core  circuit switching, packet switching 1.4 Delay, loss and throughput in Internet 1.5

Protocol layers, service models 1.6

Networks under attack: security Lecture 2 1-1

Quick Recap…

 Hardware view of Internet  Components of Internet  Structural view   Client-server model Peer-to-peer model Lecture 2 1-2

The Network Core

 Internet: mesh of interconnected routers  How is data transferred through net?

 circuit switching: dedicated circuit per call: telephone net  packet-switching: sent thru net in discrete “chunks” data Lecture 2 1-3

Network Core: Circuit Switching

 End-end resources reserved for “call”    dedicated bandwidth resources: no sharing circuit-like (guaranteed) performance call setup required Lecture 2 1-4

Network Core: Circuit Switching

 Total network resources (e.g., bandwidth) divided into “pieces”  pieces allocated to each call  resource piece

idle sharing)

if not used by owning call (no  dividing link bandwidth into “pieces”… HOW?

  frequency division multiplexing (FDM) • Users use different frequency channels time division multiplexing (TDM) • Users use different time slots Lecture 2 1-5

Circuit Switching: FDM and TDM

FDM Example: 4 users frequency time TDM frequency Lecture 2 time 1-6

Numerical example 1

 You need to send a file of size 640,000 bits to your friend. You are using a circuit-switched network with TDM. Suppose, the circuit-switch network link has a bit rate of 1.536 Mbps (1Mb = 10 6 bits) and uses TDM with 24 slots. How long does it take you to send the file to your friend?

Let’s work it out!

Lecture 2 1-7

Packet Switching

A 100 Mb/s Ethernet B 1.5 Mb/s queue of packets waiting for output link D E C Lecture 2 1-8

Network Core: Packet Switching

Circuit switching Bandwidth division into “pieces” Dedicated allocation Resource reservation each end-end data stream divided into packets  user A, B packets share network resources   each packet uses full link bandwidth resources used as needed Lecture 2 resource contention:  aggregate resource demand can exceed amount available  store and forward: packets move one hop at a time  Node receives complete packet before forwarding  congestion: packets queue, wait for link use 1-9

Packet switching versus circuit switching



Packet switching allows users to use the network dynamically!

 resource sharing  simpler, no call setup  With excessive users:  Excessive congestion  packet delay and loss

How do delay and loss occur in Internet/network?

Lecture 2 1-10

How do delay and loss occur?

packets queue in router buffers   store and forward: packets move one hop at a time  Router receives complete packet before forwarding packets queue, wait for turn… DELAY A B Lecture 2 1-11

Four sources of packet delay

 1. nodal processing:   check bit errors determine output link  2. queueing   time waiting at output link for transmission depends on congestion level of router transmission A propagation B nodal processing queueing Lecture 2 1-12

Delay in packet-switched networks

3. Transmission delay:  R=link bandwidth (bps)   L=packet length (bits) time to send bits into link = L/R 4. Propagation delay:    d = length of physical link s = propagation speed in medium (~2x10 8 m/sec) propagation delay = d/s A transmission Note: s and R are very different quantities!

propagation B nodal processing 1-13

Total delay

d

total 

d

proc 

d

queue 

d

trans 

d

prop     d proc  = processing delay typically a few microsecs or less d queue  = queuing delay depends on congestion d trans  = transmission delay = L/R, significant for low-speed links d prop  = propagation delay a few microsecs to hundreds of msecs Lecture 2 1-14

Numerical example 2

L A R B  Example: A wants to send a packet to B. The packet size is, L = 7.5 Mb (1 Mb = 10 propagation delay.

6 bits). The link speed is, R = 1.5 Mbps. How long does it take to send the packet from A to B? Assume zero Let’s work it out!

Lecture 2 1-15

Numerical example 3

L A R R B  Example: A wants to send a packet to B. The packet size is, L = 7.5 Mb (1 Mb = 10 propagation delay.

6 bits). The link speed is, R = 1.5 Mbps. How long does it take to send the packet from A to B? Assume zero Let’s work it out!

Lecture 2 1-16

Numerical example 4

L A R R R B  Example: A wants to send a packet to B. The packet size is, L = 7.5 Mb (1 Mb = 10 propagation delay.

6 bits). The link speed is, R = 1.5 Mbps. How long does it take to send the packet from A to B? Assume zero  What if there are three packets from A?

Let’s work it out!

Lecture 2 1-17

Packet loss

   queue (aka buffer) preceding link in buffer has finite capacity packet arriving to full queue dropped (aka lost) lost packet may be retransmitted by previous node, by source end system, or not at all A buffer (waiting area) packet being transmitted B packet arriving to full buffer is lost Lecture 2 1-18

Throughput



Throughput:

rate at which information bits transferred between sender/receiver R s R s R s R R c R c R c Lecture 2 1-19

Numerical example 5: Throughput

R s R c R s R c C B R s A R c  Example: A has requested for a packet (size 640,000 bits) from server B. The packet will come through an intermediate router C. It takes 0.1 second for the packet from B to C and 0.4 seconds from C to A. (Note: 1Mb=10 6 bits). Assume zero propagation delay.

   What is the throughput from B to C? What is the throughput from C to A? What is the average throughout from B to A? Let’s work it out!

Lecture 2 1-20